High frequency sealer



Nov. 15, 1955 E. MITTELMANN HIGH FREQUENCY SEALER- Filed July 15, 1949 INVENTOR. film/WV United States Patent HIGH FREQUENCY SEALER Eugene Mittehnann, Chicago, Ill., assignor to United his-- cuit Company of America, Chicago, llll., a corporation of Delaware Application July 15, 1949, Serial No. M4398 Claims. (Cl. 53--154) This invention relates to the sealing of cartons and the like and more particularly to such sealing by the use of high frequency electrical currents.

In the sealing of cartons such, for example, as common breakfast. food packages, a line of glue is applied to the top surface of each of the pair of narrow flaps at each end of a carton. A line of glue is also applied to the top or outer surface of the inner of each pair of wide fiaps'at each end of the package. The two narrow flaps are folded into position with the inner of the pair of wide flaps on top of them, and then the outer of the pair of wide flaps is folded into position. The glue if left to dry naturally dries rather slowly and the cartons must accordingly be heldclosed in some way for an extended period.

In order to speed the drying of the glue, it has been proposed to heat it in some manner. The application of a hot plate or the like to the exterior of the flaps will heat the glue, but a great amount of heat Will be wasted in the flaps and will pass into the carton and perhaps damage the contents. Furthermore, if the plate or other structure for applying heatexternally is sufficiently hot to dry the glue rapidly, the flaps are apt to be charred or burn.

To obviate these and other difficulties, I have proposed to heat the glue by means of high frequency dielectric heating. When my apparatus is used in conjunction with a glue of a snfiiciently high power factor on an ordinary cardboard carton, the heating is selective so that most of the heat is dissipated in the glue and very little in the flaps. I

According to my invention, a set of electrodes is located adjacent each side of some conveying means carrying a succession of cartons to be sealed. Each set of electrodes consists of a number of parallel bars having altenate bars of unlike radio frequency potential. The two sets of electrodes are spaced a proper distance from one another so that they will press lightly against the ends of the carton and hold the flaps in closed position while heating the glue. if thecartons are to be moved at any considerable speed over the conveying system, the heating of the glue must be quite rapid so that .the glue will be set when the carton leaves the electrode system. Accordingly, it is necessary to provide fairly high potential to the electrodes. As is wellknown, when rectangular bar-like electrodes are utilized, the electrostatic field between them is not homogeneous but fringes outward from the edges of the electrodes. The fringed part of the field is what is utilized to heat the glue on the carton flaps. When I attempted to utilize such rectangular bar-like electrodes with the flat side (i. e. the longer side of the cross section) against the flaps, I found that corona formed between the electrodes and there were reepated flash-overs. This was due to the concentrated field between the narrow edges of the electrodes. I found that by rotating the bars about their respective axes so that the narrow edges were against the flaps, heating was satisfactory and the field between the elcetrodeswas no Patented Nov. 15, 1955 longer sufficiently concentrated to cause corona and flashovers.

Constant power must be delivered from the oscillator to the glue on the carton flaps if uniform heating is to be obtained. If the heating were not uniform, some of the cartons would be imperfectly sealed while the glue and flaps on other cartons would be over-heated so as to char and perhaps burn them. Different cartons will present a different load to the oscillator due to differences in the dimensions and consistency of the flaps and glue, and this normally leads to uneven power transferred to the load and consequent defections, as previously noted, in the sealing operation. As is well known, there will also be deleterious defects in the oscillator, such as the dissipation of unnecessary power on the anode of the oscillator tube, which materially shortens its life. In order to maintain constant power transferred to the load, I have provided a tuned electrode system and have provided automatic means responsive to changes in the plate current of the oscillator to retune the electrode system if it becomes detuned, it being understood that the oscillator voltage is maintained constant so that changes in plate current are indicative of changes in the tuning of the electrode systern.

My invention will be more readily understood in its particulars from the following description when taken in conjunction with the accompanying drawing in which:

Fig. 1 is a schematic diagram of my high frequency carton sealer including a schematic diagram of a carton and conveying system;

Fig. 2 is a schematic diagram of the compensating network maintaining constant power transferred to the load; and

Fig. 3 is a plot of theoscillator plate current as a function of the tuning of the electrode system.

Referring first most particularly to Fig. 1, the high frequency oscillator or generator generally designated as 4 includes an oscillator tube 6 and a tuned output tank circuit 8 including an inductance It), a fixed capacitor 12, and a variable capacitor 14 by which the oscillator may be tuned. The feedback circuit of the oscillator includes a capacitor 16 and inductance 18 connected between the anode and cathode of the oscillator tube, with a coupling capacitor 20 and grid resistor 22 connected in a well known manner. Filament power for the oscillator tube 6 is provided by a filament transformer 24 having a center tapped secondary with the center tap connected to the negative terminal of the B+ voltage supply 26. The positive terminal of the voltage supply 26 is connected through a resistor 28 to the oscillator tank circuit. The junction between the resistor 28 and the oscillator tank circuit is grounded, as at 36, to preclude any possibility of a high D. C. voltage appearing on the carton sealing electrodes.

A carton 32 is shown on a conveying system indi ca.ed schematically as an endless belt 34 passing over a roller 36. A set of electrodes 38, is provided at each end of the carton and spaced a sufficient distance to press lightly against the carton flaps and hold them in closed position while heating the glue on the flaps. The electrodes in the preferred embodiment are elongated rectangular bars with the narrow edges of the bars (i. e. the short dimension of the rectangular cross section) pressing against the flaps. Alternate bars as 42, 44 in the electrode set 38 are connected in parallel to the extremities of the secondary 46 of a coupling transformer 48. The electrode set 38 is tuned by means of a variable capacitor 50 and an additional inductance 52 connected in parallel across the secondary 46 of the transformer 48. The electrode set 40 is similarly connected in two parallel groups to another secondary 54 of the transformer 48 and is tuned by means of a variable capacitor 56 and an additional parallel inductance 58. It is to be understood that it would be possible to connect both sets of electrodes to the same secondary and utilize a single variable capacitor and additional inductance for tuning them. The capacitors 50, 56 are provided with a mechanical coupling indicated as dashed lines 60 to a tuning motor to be described later. The primary 62 of the transformer 48 is connected across a transmission line 64 which is desirably of the co-axial type and is terminated at its other extremity by an inductance 66 coupled to the tank circuit inductance 10. The transmission line 64 is matched in the well known manner to the oscillator and load circuit.

A compensating system for maintaining constant power transferred to the load is shown schematically in Fig. 2 and is generally indicated as 68. A potentiometer 70 having a variable tap 72 is connected in parallel with the resistor 28 of the oscillator plate circuit between the positive terminal of the D. C. voltage supply and the ground connection 30 of the tank circuit 8 so that the voltage drop in the potentiometer will be proportional to the plate current drawn by the oscillator. A pair of vacuum tubes 74 and 76 have their anodes connected through the windings 78 and 80 of a polarized relay and from there through a common circuit including a switch 82 to a plate voltage supply schematically indicated as a battery 84. The tubes shown are of the pentode type and have their screen and suppressor grids connected in parallel to their respective plates or anodes to operate as triodes. The cathodes of the tubes 74 and 76 are connected together to a resistor 86 which has its other end connected to the negative terminal of the battery 84. The junction between the cathode resistor 86 and the battery 84 is also connected to ground through the parallel combination of a standard reference voltage, which may be a voltage regulator glow tube 88, and a potentiometer 90 having a variable tap 92. The control grid of the tube 74 is connected through a resistor 94 to the variable tap 72 of the potentiometer 70 and the control grid of the tube 76 is connected through a resistor 96 to the variable tap 92 of the potentiometer 90. The polarized armature 98 may be pivotally supported at its mid point adjacent the windings 78 and 8t), and has one terminal which is shown as the positive terminal of a voltage supply 100 com nected to it. The negative terminal of this voltage supply is connected directly to a tuning motor 102 which is connected to the previously mentioned tuning shaft 60. Contacts 104 and 106 are provided adjacent the extremities of the armature 98 so that as the armature moves it will contact one or the other of them to drive the motor in the proper direction to retune the electrode system.

Fig. 3 indicates the plate current of the oscillator as a function of the tuning of the electrode system, the plate current of course being highest when the electrode system is tuned to the exact frequency of the oscillator. If the operating point is selected for a particular position on one side of the curve, such as I1, the plate current will tend to vary between two extremes, such as I2 and I3 as the tuning of the electrode system changes due to variations in cartons and as moisture evaporates from the cartons due to heat.

Prior to operation, the compensating system shown in Fig. 2 may be balanced by moving the taps 72 and 92 so that the polarized armature 98 will be contacting neither of the contacts 104, 106. Following this, if a change occurs in the tuning of the electrode system, such as to bring the electrode system more nearly into tune with the frequency of the oscillator, the plate current will be increased toward Is. The voltage drop across the potentiometer 70 will thus be increased and consequently the potential on the control grid of the tube 74 will increase. This causes a greater plate current to flow through the winding 78 of the polarized relay and also through the cathode resistor 86. Due to the glow tube 88, the voltage across the potentiometer 90 remains constant and consequently the absolute potential of the control grid of the tube 76 remains unchanged. However, the potential of this control grid relative to the cathode of the tube 76 becomes more negative due to the increased voltage drop in the cathode resistor 86, thus causing a decreased flow of current through the winding of the polarized relay and emphasizing the plate current variations of the oscillator by a push-pull action and unbalancing the polarized relay from its equilibrium so that the armature contacts one of the contacts, for example 166. This causes the tuning motor to operate the tuning shaft 60 in the proper direction to tune capacitors 50 and 56 to return the plate current to its initial value, Ii, at which time the windings 78 and 80 of the polarized relay will again carry substantially equal currents and the armature 98 will return to its equilibrium position where it is not contacting either of the contacts 104 or 106. From the foregoing description, it is believed that the operation of the compensating or tuning circuit 68 is obvious when the plate current decreases toward the value lg to drive the tuning motor in the opposite direction and retune the electrodes to return the plate current to the position I1.

It is apparent that I have herein presented a means for sealing cartons by the utilization of high frequency currents to dry or set a glue on its flaps or to electroni cally weld flaps on a thermo-plastic carton covering. It is apparent that the electrode system is automatically kept tuned by means responsive to changes in oscillator plate currents from a given initial value so that each carton is heated a desired uniform amount and the oscillator remains in stable operation.

Although for purposes of illustration I have shown and described a particular embodiment of my invention, it is to be understood that I do not wish to be limited by this embodiment, but intend my invention to encompass every form that falls fairly within the spirit and scope of the appended claims.

I claim:

1. A high frequency carton sealer comprising a high frequency oscillator having an output circuit, a pair of electrode sets, each electrode set comprising a plurality of pairs of bar-like electrodes of rectangular cross-section carired in parallel spaced relationship with the narrow longitudinal edges of the electrodes in each set facing the narrow longitudinal edges of the electrodes in the other set, means for coupling the first of said electrode sets to said output circuit, means for coupling the second of said sets to said output circuit, each of said coupling means being connected to energize alternate bars in an electrode set oppositely to one another, individually operable impedance elements for tuning each of said electrode sets, concurrently operable variable impedance elements for concurrently tuning both of said electrode sets, means for feeding a succession of articles past said electrode sets, and means coupled to said concurrently operable variable impedance elements and responsive to changes in the plate current of said oscillator to vary said concurrently operable elements.

2. A high frequency carton sealer comprising a high frequency oscillator having an output circuit, a pair of sets of electrodes, each electrode being in the shape of a rectangular bar with its narrow longitudinal edge positioned to contact a succession of cartons and all of the electrodes of each set being substantially parallel and coplanar, means for moving a succession of cartons between said electrode sets and perpendicular to the individual electrodes thereof and in contact therewith to hold the carton flaps closed while supplying heat to glue thereon, means coupling said electrode sets to said output circuit, said coupling means being connected to energize alternate electrodes in each set of electrodes oppositely to each other, concurrently operable variable im- 5 pedance elements for concurrently tuning a plurality of said electrode sets, and means-coupledto'said concurrently ripperable variable impedance elements and responsivev to (changes -in..the plate current of said oscillator to vary tisaid concurrently operable elements.

3. A high frequency carton sealer comprising a high t'frequency oscillator having an output circuit,-a tunable -.'electrode .system, a variable reactance' for tuning said r-electrode system, means coupling said electrode system v:said output circuit, a standard reference voltage, a

voltage developed as a function of the plate current of said oscillator, an electron vacuum tube biased proporrrtional to said latter voltage to vary the plate current of i said electron tube with variations .in' said latter voltage, nan: electron vacuum tube biased proportional to said sstandard: reference voltage, a common cathode circuit for saidelectron vacuum tubesssouthata change in the ggplaterurrent of said first tube causes an inverse change rinatthe platecurrent of said second. tube, a pair Of! relay windingsconnected to one another and respectively/no the plates of said electron tubes, means connected to the junction of said relay windings supplying plate power zto said: electron .tubes,.a relay armature/pivoted substantially at its mid'pointrand.havingits-opposite ends posi tioned for attraction respectively by said relay coils, and a'drive'motor'controlled bysaid armature and coupled to said variable reactances to-maintain. s'aidelectrode system tuned.

4. A high frequency carton sealer comprising a high frequency oscillator having an output circuit, a tunable electrode system having a plurality of bar-like electrodes each having relatively wide faces and narrow edges, alternate electrodes being oppositely polarized and the wide faces of adjacent electrodes confronting one another, means for conveying a succession of cartons past the narrow edges of said electrodes, at variable reactance for tuning said electrode system, means coupling said electrode system to said output circuit, a standard reference voltage, a voltage developed as a function of the plate current of said oscillator, an electron tube biased proportional to said latter voltage to vary the plate current of said electron tube with variations in said latter voltage, an electron tube biased proportional to said standard reference voltage, a common cathode circuit for said electron tubes so that a change in the plate circuit of one tube causes an inverse change in the plate current of the other tube, a pair of relay windings connected to one another and respectively to the plates of said electron tubes, means connected to the junction of said relay windings for supplying plate power to said electron tubes, a relay armature pivoted substantially at its mid point and having its opposite ends positioned for attraction respectively to said relay coils, and a drive motor controlled by said armature and coupled to said variable reactance to maintain said electrode system tuned.

5. A high frequency carton sealer comprising a high frequency oscillator having an output circuit, a set of elongated electrodes each having a substantially rectangular cross section, said electrodes being positioned in parallel spaced relation with one of the short sides of the rectangular cross section of each of said electrodes lying in substantially a common plane, means for coupling said set of electrodes to said output circuit, said coupling means being connected to energize alternate electrodes oppositely to one another, a variable impedance element for tuning said electrode set, and means coupled to said variable impedance element and responsive to changes in the plate current of said oscillator to vary said variable impedance element.

6. A high frequency carton sealer comprising a con veyor adapted to carry articles spaced thereon having heat .sealable flaps disposed on one side thereof, a high frequency oscillator having an output circuit, a set of elongated electrodes positioned adjacent said conveyor and each having a substantially rectangular cross section,- said electrodes being positioned in parallelspaced relation with one of the short sides of the rectangular cross section of, each of said electrodes, lying in substantially a common plane, the electrode faces lying in the common plane pressing the heat scalable flaps on said article-s to the closed position for sealing, the width of said electrodes being small compared to the length of the flap passing andeach having a substantially rectangular. cross section,

said electrodes ,being positioned in parallel spaced relation .with one oftheishort sides of the rectangularcross section of each of said electrodes lying in substantially a common plane, the clectrodefaces lying in the common planepressing the heat scalable flaps on said articies to the closed position for sealing, the width of said electrodes being small compared to the length of the flap passing said electrodes, the spacing between said articles rapidly varying the impedance reflected by said set of electrodes, meansfor coupling said set of electrodes to said output circuit, said coupling means being connected to energize alternate electrodes oppositely to one another, a variable impedance element for tuning said set of electrodes, and means coupled to said variable impedance element and responsive to changes in the plate current of said oscillator to vary said variable im pedance element whereby to compensate for the varying impedance reflected by said set of electrodes as the articles pass said electrodes.

8. A high frequency carton sealer comprising a high frequency oscillator having an output circuit, two spaced apart sets of elongated electrodes, each electrode having a substantially rectangular cross section, the electrodes of each set being positioned in parallel spaced relation with one of the short sides of the rectangular cross section of the electrodes in each set lying in substantially a common plane, said short sides of the rectangular cross section of one set of electrodes confronting and being spaced from the short sides of the other set of electrodes, and means for coupling each said set of electrodes to said output circuit, said coupling means being connected to energize alternate electrodes in each set oppositely to one another.

9. A high frequency carton sealer comprising a conveyor for carrying cartons having heat scalable end flaps on opposed ends thereof, the ends carrying the heat sealable flaps of saidarticles being disposed on opposite sides of the conveyor along a line perpendicular to the direction of travel of said conveyor, a high frequency oscillator having an output circuit, two sets of elongated electrodes positioned adjacent said conveyor to bear against the flaps of said spaced articles, each electrode being substantially rectangular in cross section, the electrodes in each set being positioned in parallel spaced relation with one of the short sides of the rectangular cross section of each of said electrodes lying in substantially a common plane, the electrode faces in each of said common planes pressing against the flaps of an adjacent end of an article to close and seal the flap, the width of each electrode being small compared with the length of the flaps passing said electrodes, and means for coupling each of said sets of electrodes to said output circuit, said coupling means being connected to energize alternate electrodes in each set oppositely to one another.

10. A high frequency carton sealer comprising a conveyor for carrying cartons having heat scalable end flaps on opposed ends thereof, the ends carrying the heat sealable fiaps of said articles being disposed on opposite sides of the conveyor along a line perpendicular to the direction of travel of said conveyor, a high frequency oscillator having an output circuit, two sets of elongated electrodes positioned adjacent said conveyor to bear against the flaps of said spaced articles, each electrode being substantially rectangular in cross section, the electrodes in each set being positioned in parallel spaced relation with one of the short sides of the rectangular cross section of each of said electrodes lying in substantially a common plane, the electrode faces in each of said common planes pressing against the flaps of an adjacent end of an article to close and seal the fiap, the Width of each electrode being small compared with the length of the flaps passing said electrodes, means for coupling each of said sets of electrodes to said output circuit, said coupling means being connected to energize alternate electrodes in each set oppositely to one another, the spacing between articles causing a rapid varying of the impedance reflected by each of said sets of electrodes back toward said output circuit, a variable impedance element for tuning each or" said sets of electrodes, and means coupled to each of said variable impedance elements and responsive to changes in the plate current of said oscillator to vary said variable impedance elements whereby to compensate for the varying impedance reflected by said electrodes as the spaces between the articles pass the electrodes thereby reflecting a. substantially constant load back into said output circuit.

References Cited in the file of this patent UNITED STATES PATENTS 1,683,558 Kolster Sept. 4, 1928 2,288,269 Crandell June 30, 1942 2,289,670 McClellan July 14, 1942 2,415,799 Reifel et al. Feb. 11, 1947 2,416,172 Gregory et al. Feb. 18, 1947 2,433,067 Russell Dec. 23, 1947 2,467,285 Young et al Apr. 12, 1949 2,470,443 Mittelmann May 17, 1949 2,473,188 Albin June 14, 1949 2,491,422 Livingston Dec. 20, 1949 2,503,779 Story Apr. 11, 1950 2,528,722 Collins Nov. 7, 1950 2,535,413 Hart et al Dec. 26, 1950 2,546,004 Kinn Mar. 20, 1951 2,551,757 Mittelmann May 8, 1951 2.606.856 Hurrey et a1 Aug. 12, 1952 2,631,642 Richardson Mar. 17, 1953 2,632,091 Hagopian Mar. 17, 1953 FOREIGN PATENTS 535,719 Great Britain Apr. 18, 1941 OTHER REFERENCES Load Rematching in Electronic Heating, Electronics, Febuary 1945, pages 110-115, particularly pages 114 and 115. Copy in Scientific Library. 

